The present invention relates to medical screws, and more particularly to pedicle screw assemblies.
Pedicle screws are commonly used to assist in various forms of spinal fusion surgery. Spinal fusion surgery can be performed to correct a variety of spinal conditions caused by damage to or degeneration of a segment of the spine. Spinal fusion procedures typically involve fusing together segments of the spine to prevent conditions resulting from their relative movement. In a typical spinal fusion procedure, a portion of the spine is removed and bone grafts are implanted to form a bridge that joins the spine segments on opposite sides of the removed portion. Pedicle screws are used during this process to hold the spine segments to be fused so that the bone grafts can heal and form a solid bridge. More specifically, pedicle screws are installed in spinal segments to provide anchor points that allow the spinal segments to be fixedly connected with rods. For example, in a common form of spinal fusion surgery, pedicle screws are installed in a series of consecutive spine segments. One or more rods are installed between the screws to secure the spinal segments with respect to one another. Once installed the consecutive spinal segments remain fixed with respect to one another, thereby allowing bone grafts to fuse to the spine, forming a solid bone.
In one example, spinal fusion surgery can be used to correct a spinal condition caused by spondylolisthesis in which broken bones or weakened joints allow a vertebra to slip forward and pinch a nerve root. In the context of this particular procedure, the lamina of the damaged vertebra and any remaining bone that may be pinching the nerve roots are removed. Bone grafts are then added to the sides of the spine joining the vertebrae above and below the damaged vertebra. Pedicle screws are installed in the vertebrae above and below the damaged vertebra. Rods are installed between the pedicle screws to prevent relative movement between the two spine segments, which allows the bone grafts to fuse to the spine, forming solid bone.
A typical pedicle screw includes an assembly with a rod retainer (or cup) pivotally mounted to the head of a screw. In conventional applications, the screw is installed in a spinal segment and the rod retainer remains exposed on the exterior of the spinal segment. The rod retainer includes structure that allows a rod to be fixedly secured to the pedicle screw assembly. For example, a conventional rod retainer may have a rod retainer slot and a set screw for securing the rod within the rod retainer slot. In use, a single rod may be secured between two or more pedicle screw assemblies to tie together the pedicle screw assemblies. The rod retainers are pivotally mounted to the heads of the screws to facilitate linking of the pedicle screw assemblies with a rod. More specifically, once the pedicle screws are installed in the spine, the rod retainers can be pivotally moved about the screw heads to bring the rod retainer slots into alignment to allow then to be joined by a single rod.